Ancient "Giant Virus" Revived From Siberian Permafrost

Climate change could release more ancient viruses. Is there a risk to humans?

Buried deep in the Siberian permafrost and untouched for over 30,000 years, researchers have discovered what is thought to be the newest representative of what are loosely known as "giant viruses."

A team led by Jean-Michel Claverie and Chantal Abergel of Aix-Marseille University in Marseille, France, made the discovery of the previously unknown virus, which has been dubbed Pithovirus sibericum and can be revived in the lab.

Lest Siberian excavation makes you think of frozen woolly mammoths, make no mistake: These giants are still decidedly microscopic. But in the diminutive world of viruses they're larger than normal specimens, measuring 1.5 microns in length and 0.5 microns in diameter. The pandoraviruses, the largest viruses previously discovered, also by the team of Claverie and Abergel, measure 1 micron in length and 0.5 in diameter.

"'Giant' viruses are loosely defined as the ones that you can see under a regular microscope," explained Claverie and Abergel when contacted via email.

Large and Complex

Giant viruses also dwarf other viruses in terms of genetic complexity. The newly discovered Pithovirus contains 500 genes, and the aforementioned Pandoravirus can contain up to 2,500.

For comparison, the HIV virus contains only about 12 genes, explained James Van Etten, a professor of plant pathology at the University of Nebraska, when reached for comment. (Van Etten is an authority on viruses and edited the new study.)

Amazingly, even after more than 30,000 years embedded in ancient permafrost, when Claverie and Abergel exposed amoebas in their lab to the virus, they found that the virus was still active and quickly infected the host cell. "We use amoeba on purpose as a safe bait for capturing viruses. We then immediately verify that they are not able to infect animal/human cells," stressed the researchers.

Giant viruses are not just bigger but are hardier than others as well, said the researchers. This hardiness, along with a favorable environment, likely helped the newly discovered specimen stay intact for the thousands of years that it did. Viruses are often destroyed or rendered inactive by a number of factors, including light and biochemical degradation.

"Among known viruses, the giant viruses tend to be very tough, almost impossible to break open," said Claverie and Abergel. "Special environments such as deep ocean sediments and permafrost are very good preservers of microbes [and viruses] because they are cold, anoxic [lacking oxygen], and in the dark."

Intricate and Varied

The past decade has seen something of a renaissance in the discovery of large, genetically complex viruses, with the discovery of three distinct groups (Mimivirus, Pandoravirus, and now Pithovirus) suggesting that viruses can be much more intricate and varied than previously thought, and that giant viruses may not be especially uncommon.

The newly discovered diversity in genetic makeup and morphology among viruses leads Van Etten to surmise that different types of viruses may have evolved separately.

"The idea that all viruses evolved from one common origin, I suspect is not true," said Van Etten.

Changing Climate: Growing Threat?

If long-buried viruses can be unearthed, what else might be capable of coming to the surface? Climate change as well as industrial activities may shake up the ancient ice enough to bring potential pathogens to the surface.

"Mining and drilling means ... digging through these ancient layers for the first time in millions of years. If 'viable' [viral particles] are still there, this is a good recipe for disaster," said Claverie and Abergel.

But Edward Mocarski, a professor of microbiology at Emory University, says the risk of a virus pathogenic to humans being released from the ice is very small.

"The idea would make a great movie but is extremely unlikely unless the virus came from a frozen human being who possibly died from a virus that is no longer in circulation," said Mocarski when contacted via email.

"A very small proportion [of the viruses on Earth] represent viruses that can infect mammals and an even smaller proportion pose any risk to humans."

University of Nebraska's Van Etten agreed that such a situation was unlikely but possible with the right conditions.

"The biggest source of genes on the planet is probably from viruses, and they're just everywhere, but in general they're highly specific for the organisms that they grow in," said Van Etten.

The researchers behind the discovery, Claverie and Abergel, believe that whether or not it's likely, such a scenario remains feasible. They counsel vigilance and continued testing. As their latest research has shown, large DNA viruses may remain infectious for very long stretches of time.

"The fact that we might catch a viral infection from a long-extinct Neanderthal individual is a good demonstration that the notion that a virus could be 'eradicated' from the planet is plain wrong and gives us a false sense of security. At least a stock of vaccine should be kept, just in case," said Claverie and Abergel.

Their research will now turn to assessing how real a threat these ancient viruses pose.

"We are now doing more work to analyze the DNA content of these permafrost layers in a search for the genetic signature of viruses resembling human pathogens," said Claverie and Abergel, who stressed that they are not attempting to "revive" any such pathogenic viruses, but rather hoping to determine the potential danger.

"If we find some [human pathogens], then the risk will become more real. If not, we will be safe."